1. Field of the Invention
The present invention relates to a lithium secondary battery, which utilizes the inserting-and-extracting phenomenon of lithium ions. More particularly, it relates to an improvement on its negative electrode.
2. Description of the Related Art
Since the lithium secondary battery exhibits a high energy density, it has been put into practical application and has been available in the field of communication or information devices as cellular phones, personal computers, and video cameras, etc., are down-sized. Moreover, it is considered to apply it in the other usages, for example, a power source for an electric vehicle and an electricity storing system, etc.
Now, the main stream of the lithium secondary battery is a so-called locking-chair type lithium secondary battery, which uses a carbonaceous material as the negative electrode. In the lithium secondary battery using a carbonaceous material as the negative electrode active material accompanies a problem, so-called the retention problem, in that the lithium inserted in the negative electrode by the initial charging is trapped in the negative electrode, is not discharged in the discharging thereafter, and remains as the irreversible capacity.
The retention of the negative electrode not only results in the sudden capacity variation in the secondary battery, but also in the wasting of the positive electrode active material, because it is necessary to arrange the lithium secondary battery so that the facing positive electrode involves lithium in an excessive amount, which is equivalent to the irreversible capacity, in order not to decrease the capacity of the secondary battery. Hence, in the case of the lithium secondary battery, it is desired to elaborate a certain means for relieving the retention of the negative electrode efficiently.
The carbonaceous material exhibits a theoretical capacity of from 350 to 400 mAh/g even if it is said to exhibit a large capacity. In order to increase the capacity of the lithium secondary battery, it is needed to urgently select a large capacity negative electrode active material. Some of the large size negative electrode active materials, being in the limelight, are Al, Sn, Pb and their alloys. In particular, an Snxe2x80x94Sb base alloy is said to exhibit an ideal capacity, which exceeds 1,000 mAh/g.
However, the problem of the negative electrode retention is not only associated with the carbonaceous materials, but also with the other active materials. For example, the aforementioned Snxe2x80x94Sb base alloy only produces an effective capacity of from 550 to 600 mAh/g approximately, in contrast with the nominal theoretical capacity exceeding 1,000 mAh/g. Thus, there also exists a large irreversible capacity.
On the other hand, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 11-3,707 and Japanese Unexamined Patent Publication (KOKAI) No. 9-298,068, trials are made to use a lithium nitride expressed by a composition formula, Li3xe2x88x92xMxN, in which a transition element xe2x80x9cMxe2x80x9d substitutes for a part of the Li. In this Li3xe2x88x92xMxN, since it is already composed of lithium, no retention is present. However, in the lithium secondary battery, since LiCoO2, etc., is used generally as a pairing positive electrode active material, the composing lithium should be extracted, for example, by an electrochemical means in advance from Li3xe2x88x92xMxN. Accordingly, it is necessary to carry out the lithium extracting process. Hence, it is one of the causes that Li3xe2x88x92xMxN is inhibited from using as a negative electrode active material and accordingly from putting it into actual applications.
The inventors of the present invention thought of actively utilizing the aforementioned characteristic of Li3xe2x88x92xMxN that Li3xe2x88x92xMxN includes lithium, and discovered that the lithium involved in Li3xe2x88x92xMxN can be stored in the other negative electrode active material by compositing Li3xe2x88x92xMxN with the other negative electrode active material. The present invention is developed based on the discovery. It is an object of the present invention to provide a lithium secondary battery, whose negative electrode retention is relieved efficiently, which is free from the abrupt capacity variation, and which wastes the positive electrode active material less, by compositing two or more negative electrode active materials so as to compensate the disadvantages with each other.
It is the other object of the present invention to provide a lithium secondary battery, which has a larger capacity while relieving the negative electrode retention, by selecting the other negative electrode active materials to be composited.
A lithium secondary battery according to the present invention is characterized in that a negative electrode active material comprises: at least one primary active material selected from the group consisting of carbonaceous materials, Sn, Sn alloys, Sn oxides, Al, Al alloys, Pb, Pb alloys, Si and Si oxides; and a secondary active material comprising a lithium nitride expressed by a composition formula, Li3xe2x88x92xMxN, in which xe2x80x9cMxe2x80x9d is at least one element selected from the group consisting of transition metals and xe2x80x9cxxe2x80x9d is 0 less than xe2x80x9cxxe2x80x9dxe2x89xa60.7. Namely, the present invention is a secondary lithium battery, in which two kinds of active materials, the primary active material and the secondary active material, are composited or, in other words, are mixed to use as a negative electrode active material.
The materials listed as the primary active material are all active materials, in which the capacity difference exists between the initial charging capacity and the discharging capacity thereafter, and which take in a large amount of lithium as the irreversible capacity. On the other hand, since the secondary active material, Li3xe2x88x92xMxN, includes Li in advance, there does not exist the irreversible capacity theoretically. However, they are active materials which do not produce a large capacity unless Li is extracted in advance.
When the negative electrode is constituted by mixing the two kinds of the active materials, and when it is contacted with an electrolyte, Li extracts from Li3xe2x88x92xMxN of itself (without adding extra energy), and is inserted in the primary active material. As a result, the primary active material is filled with Li in an amount equivalent to the irreversible capacity, and becomes an active material which is free from the retention. Simultaneously, the secondary active material, from which Li extracts, becomes an active material of a large capacity. Thus, by the simple means, such as by simply compositing the active materials so as to constitute the negative electrode, it is possible to make a secondary battery according to the present invention having a favorable characteristics, secondary battery from which the negative retention is relieved efficiently, which has a large capacity, and which is free from the abrupt capacity variation. Even if the present battery has the same capacity as that of a conventional battery, the present lithium secondary battery does not require a positive active material in an excessive amount. Thus, the present lithium secondary battery is good in terms of cost as well.
In the lithium secondary battery according to the present invention, it is possible to select the primary active material from the listed substances, and to also select at least one member from the group consisting of Sn, Sn alloys and Sn oxides. Namely, the Sn base substances can be used as the primary active material.
These Sn base substances insert Li therein to make Li alloys so that they make active materials having an extremely large capacity. By thus selecting the Sn base substances as the primary active material, the present lithium secondary battery becomes a secondary battery, which has an extremely large capacity (or a large capacity density).
The lithium secondary battery according to the present invention is constituted so that the composite active material, which is made by compositing the primary active material and the secondary active material, is used as the negative electrode active material. The primary active material can include carbonaceous materials, Sn base substances, etc., and the secondary active material can include lithium nitrides a part of whose lithium sites are replaced by a transition metal. With this arrangement, it is possible to insert Li in the primary active material, due to the voltage difference, Li which is present in the secondary active material in advance. Thus, the present secondary battery makes a secondary battery, in which the retention occurred in the negative electrode is relieved efficiently. Moreover, it is possible to make a lithium secondary battery having a lager capacity by using Sn base substances as the secondary active material.